The constant progress in semiconductor technology demands for the fabrication of ever smaller devices. This development has to be accompanied by concurrent improvement in metrology capabilities, in order to monitor and control the fabrication process.
Over the last few decades, optical critical dimension (OCD) metrology has taken a pivotal role in semiconductor manufacturing process, due to its extreme sensitivity, accuracy, flexibility and speed. In order to provide adequate improvement of the metrology capabilities, OCD tools have gone through extensive improvement and refinement, and can provide today extremely accurate broadband spectral measurements and extremely high throughput.
In addition to the process of improving the basic tool characteristics, another venue by which OCD performance can be improved is through diversifying the measured information. Commonly measured optical properties are the reflectivity for different incidence angles, azimuths, polarizations and wavelengths. In addition, the relative phase between reflected TE and TM polarization components can be accessed through (e.g.) ellipsometric measurements.
Another important attribute of light scattered from a patterned structure is its spectral phase. This quantity describes the relative phase between the incident and reflected electromagnetic waves. Typically, this phase has different values for different wavelengths, incident angles\azimuths and polarizations.
Since accessing the phase directly is not possible at optical frequencies, one has to use interference effects, usually observed with an interferometer, and recover the encoded phase information from the interference effects. Most interferometers consist of a split optical path that is recombined to form interference fringes. One arm of the path is kept as a reference, and the other interacts with the sample. The interference signal from these two components is then used to extract the spectral phase.
U.S. Pat. No. 6,985,232 describes a phase-sensitive interferometeric broadband reflectometer for optically inspecting and evaluating a subject. According to this technique, a broadband optical beam is split into probe beam and reference beam portions; the probe beam is directed to be reflected by the subject; after the probe beam has been reflected by the subject, the probe beam and the reference beam are rejoined. The length of the path traveled by the probe beam or the reference beam is modulating within a predetermined range during the measurements. Then, spectroscopic analysis of the rejoined beams is performed on a per-wavelength basis at a selected set of points within the predetermined range.